A multiplicity of projection units are known from the prior art. In this case, a distinction is made, in particular, between projection units for video projection, such as, for example, liquid crystal display (LCD), digital light processing (DLP) or liquid crystal on silicon (LCoS) projection units, which are used for example for video projection or digital cinema projection, and projection units for advertizing and entertainment applications, such as, for example, moving head or graphical optical blackout (Lobo) projection units. Projection units of this type usually use a discharge lamp for generating light.
In this case, the discharge vessels of the discharge lamps and thus the arc that forms between the electrodes are positioned axially in a parabolic or elliptic reflector. The arrangement of lamp and associated reflector is also designated hereinafter as reflector-lamp arrangement. With the use of an elliptic reflector, a light integrator for homogenizing the light radiation is usually disposed downstream of said reflector. The light integrator is followed by the imaging element of the projection unit and the projection optical system. The light thus radiates from the discharge lamp, positioned in a reflector, via an aperture to a projection optical system. In the case of a DLP projector, for example, the aperture is the area of the light entrance opening of the light integrator. For years the trend here has been toward ever smaller dimensions of the projection units, the size of the aperture also being reduced. In order furthermore to be able to achieve a high system efficiency, the discharge lamps are used with ever shorter arcs, i.e. shorter electrode spacings. This trend also applies to Gobo projectors and to digital cinema projection. Moreover, discharge lamps having ever higher powers are increasingly being used.
What is disadvantageous here is that, as a result of wear of the electrodes over the service life of the discharge lamp, the emission characteristic thereof (for example the spatial position and form of the arc between the electrodes and the luminance distribution of said arc) changes, as a result of which the light intensity distribution of the emitted light also changes relative to the aperture and can shift or change both in the aperture plane and perpendicularly thereto. This in turn has the effect that that proportion of the luminous flux of the discharge lamp which enters into the aperture generally decreases significantly over the service life on account of this change and shift, even though the total luminous flux emitted by the discharge lamp hardly changes.